1. Field of the Invention
This invention relates to a process of producing reinforced flat sheet, tubular and contoured laminates from crosslinkable thermoplastic olefin polymer material.
2. Description of the Prior Art
Thermoplastic polyolefin resins, by virtue of their hydrocarbon structure, have a low dielectric constant and good electrical insulating properties. As a result, such resins have heretofore been used as wire and cable insulation and in reinforced compositions for electronics applications. In some applications of the latter type, the electrical loss of radio-frequency energy is not important and there are well-known dielectric materials, such as phenolic paper, epoxy paper, epoxy cotton, and several other grades of reinforced laminates which are presently used by the electrical industry. Each of these materials varies in usefulness depending upon cost, dielectric properties, and operating temperatures.
There has been a recent trend toward the use of microwave devices and a need has arisen for dielectric products having the lowest possible electrical loss in terms of dissipation factor and loss tangent and which are flame-retardant, drip proof, resistant to chemicals and cold-punchable. The materials presently used by the electrical industry described above are limited in electrical properties at microwave frequencies. This characteristic thus limits their usefulness in many such applications.
Polyolefin resins have desirable electrical properties as a homogeneous dielectric material. These resins have been combined with a suitable reinforcing substrate, such as glass fabric, to produce laminates having some of the thermosetting properties of materials like the epoxy resins. The use of such a reinforcing substrate improves the physical properties of the resin and produces a reinforced polyolefin possessing good dimensional, tensile, flexural, bursting and tear strengths.
Polyolefin resins have a relatively low softening point and poor heat resistance, however. These characteristics render the resins unsuitable for the majority of electrical applications where the addition of metallic foils followed by some form of soldering is required, such as, for example, in printed circuit board fabrication. As a result, although such laminates have the desired electrical characteristics and adequate strength, exposure of the laminates to elevated temperatures generally causes delamination of the laminate. Furthermore, such resins drip when heated and are not flame-retardant.
One solution to the foregoing problem is to first crosslink the polymer and then combine it with the reinforcing substrate. Crosslinking can be achieved by irradiating sheets of crosslinkable polyolefin with high-energy electrons after extruding the sheets and prior to forming the laminate. Reinforced polyolefin laminates of this type have been fabricated by combining a thermoplastic olefin polymer such as polyethylene crosslinked by irradiation and a reinforcing substrate such as glass fiber by applying the polymer to the reinforcing substrate and then heating the laminate to fuse the resin to the substrate. Irradiation of the polymer sheet material and the subsequent assembly of the sheet material into a laminate by combining the sheet material with a reinforcing substrate and heating the combination under pressure to achieve consolidation is, however, both expensive and difficult to maintain in continuous production.
Crosslinking of the polyolefin used to form the laminate can also be chemically achieved by the use of heat and a catalyst. Such crosslinking has, however, heretofore produced a non-homogeneous, variable dielectric polyolefin material.